The present invention relates to a solid-state image sensor, more particularly, to a complementary metal-oxide semiconductor (CMOS) image sensor including small pixels covered with light absorbing color filters.
A typical pixel of a modern CMOS image sensor includes a photodiode, more specifically, a pinned photodiode, and four transistors. The photodiode collects photo-generated charge that is later transferred onto a floating diffusion (FD) node at a suitable moment by a charge transfer transistor. The FD node functions as a charge detection node. Prior to the charge transfer, the FD node needs to be reset to a suitable reference voltage. The reset causes kTC noise, which would be normally added to a signal appearing on the FD node. Thus, it is necessary to read the voltage on the FD node twice, the first time before the charge transfer, and the second time after the charge transfer. This operation is called CDS (Correlated Double Sampling). The CDS operation allows sensing of only the voltage difference on the node caused by the transferred charge from the photodiode.
A source follower (SF) transistor senses the voltage on the FD node through a gate of the SF transistor connected to the FD node, a drain thereof connected to a power voltage (Vdd) terminal, and a source thereof connected to a common column sense line via addressing transistor. For this reason, incorporating 4 transistors in each pixel of a standard CMOS image sensor is generally necessary. U.S. Pat. No. 5,625,210 issued to Paul P. Lee et al. in the name of “Active Pixel Sensor Integrated with Pinned Photodiode” describes one exemplary 4T pixel circuit with a pinned photodiode.
In modern CMOS sensor designs, the circuitry for several photodiodes may be shared as can be found exemplarily in U.S. Pat. No. 6,657,665 B1, issued to R. M. Guidash et al., entitled “Active Pixel Sensor with Wired Floating Diffusions and Shared Amplifier.” In this patent application, a dual pixel includes two photodiodes located in adjacent rows of a sensor image array and sharing the same circuitry.
The color sensing in most modern CMOS image sensors is accomplished by placing suitable color filters over the photodiodes as is shown in FIG. 1. A blue color filter 101 absorbs green and red light and lets only the blue light photons to enter the photodiode area below. Similarly, a green color filter 102 absorbs blue and red light and lets only the green light photons to enter the silicon bulk below. Reference numeral 103 represents a red color filter. Blue light and green light photons have high energy and thus, are generally absorbed very quickly within a depth Xg defined from the surface of the silicon bulk to a certain region 104 thereof. On the other hand, red light photons have low energy and penetrate a region deeper than the above region 104. More specifically, before generating any photoelectrons, the red light photons can penetrate to an interface 105 between an epitaxial substrate region, located at a depth Xepi, and a highly doped P+-type substrate 106. Reference letter ‘Xr’ denotes a depth of the interface 105 from the surface of the silicon bulk (i.e., highly doped P+-type substrate 106).
When electrons 107 are generated in the highly doped P+-type substrate 106, the electrons 107 recombine very quickly with the holes located in the highly doped P+-type substrate 106 and cannot be collected in the “red” photodiode. Those electrons 108, on the other hand, which are generated in an un-depleted epitaxial layer 109, have much longer lifetime than the electrons 107, and diffuse freely in the un-depleted epitaxial layer 109 both laterally and vertically until the electrons 108 reach the boundary of depletion regions 110. The boundary of the depletion regions 110 is located at a depth Xd1 from the surface of the silicon bulk.
When electrons 111 enter the depletion regions 110, the electrons 111 are quickly swept into respective photodiode potential wells located in regions where N-type doped layers 112 are formed. The photodiodes are formed close to the surface of the silicon bulk by the N-type doped layers 112 and P+-type pinning layers 113. This structure is called the pinned photodiode. The P+-type pinning layers 113 each extend along the sides and the bottom of respective shallow trench isolation (STI) regions 114, each formed by etching the silicon bulk, to separate and isolate the photo sites and the corresponding electrical circuits from each other. The STI regions 114 are filled with silicon dioxide. The silicon dioxide also covers the photodiode surface area and extends under transfer gates 117. Reference numeral 115 and 116 respectively represent the silicon dioxide filling the STI regions 114 and the silicon dioxide extending under the transfer gates 117 while covering the photodiode surface area. The transfer gates 117 are formed of polycrystalline silicon.
When a suitable bias is applied to each of the transfer gates 117 via corresponding connections 118 (shown only schematically), electron charge stored in the photodiode potential wells is transferred onto respective FD nodes 119 formed by doping N+-type dopants. The FD nodes 119 usually experience a voltage change. This voltage change is then sensed by suitable amplifiers (SFs), which are connected individually to the FD nodes 119 by respective wires 120 (also shown only schematically). The voltage change represents a desired signal. The photodiodes and the transfer gates 117 are typically covered by another layer 121, formed by silicon dioxide or multiple layers of silicon dioxide, and other transparent films before color filters are deposited on the top. Microlenses (not shown in the drawing) are then also deposited on top of the blue, green and red color filters 101, 102 and 103 to focus the light on the surface area of the photodiodes.
As can be easily understood from FIG. 1, those electrons generated by the red light in the un-depleted epitaxial layer 109 can also diffuse laterally and enter the depletion regions 110 of the neighboring photodiodes. This phenomenon often causes unwanted color crosstalk, since the red light-generated electrons usually end up in wrong photodiode potential wells of the “green” or “blue” photodiodes. This color crosstalk may be pronounced in small size pixels where the lateral dimension of the pixel is less than 2 μm, while the vertical dimension remains on the order of 5 μm. The color crosstalk can be reduced by decreasing the thickness of the epitaxial layer (i.e., the depth Xr of the interface 105) and thus, reducing the thickness of the un-depleted epitaxial layer 109 or extending the boundary of the depletion regions 110 located at the depth Xd1 to a depth Xd2.
However, the above-described two approaches may have some limitations. The shallow epitaxial thickness causes too many of the red light electrons to be generated in the highly doped P+-type substrate 106 and thus recombined with the holes in the highly doped P+-type substrate 106. As a result, the red light electrons may not contribute to the signal. It is usually desirable to have the epitaxial thickness on the order of 5.0 μm or larger to have a good “red” light response.
The thick depletion that extends all the way to the interface 105 may also cause limitations. The low doping of the epitaxial layer that is necessary to accomplish the thick depletion may increase the dark current generation, and may lead to the discontinuity and separation of the P+-type pinning layers 113 located near the surface from the highly doped P+-type substrate 106 as indicated by the separated depletion layer boundaries 122 for this level of epitaxial doping. When the discontinuous and separated P+-type pinning layers 113 are observed, it is necessary to provide other electrical connections to the P+-type pinning layers 113 by some other means such as metal wires placed over the top of the pixels. These electric connections may reduce the pixel aperture efficiency and consequently the final pixel Quantum efficiency.